Treat parent ids as strings

Prior to this change graph.parent(x) could return a non-string id, which
is inconsistent with how node ids are treated in all other parts of the
API. This change ensures that parent ids are coerced to strings.

lib/graph.js

@@ -198,6 +198,8 @@ Graph.prototype.setParent = function(v, parent) {
   if (_.isUndefined(parent)) {
     parent = GRAPH_NODE;
   } else {
+    // Coerce parent to string
+    parent += "";
     for (var ancestor = parent;
          !_.isUndefined(ancestor);
          ancestor = this.parent(ancestor)) {

test/graph-test.js

@@ -303,6 +303,14 @@ describe("Graph", function() {
       expect(_.sortBy(g.children())).to.eql(["a", "parent"]);
     });
 
+    it("uses the stringified form of the id", function() {
+      g.setParent(2, 1);
+      g.setParent(3, 2);
+      expect(g.parent(2)).equals("1");
+      expect(g.parent("2")).equals("1");
+      expect(g.parent(3)).equals("2");
+    });
+
     it("preserves the tree invariant", function() {
       g.setParent("c", "b");
       g.setParent("b", "a");